1. Field of the Invention
The present invention relates to a light source system, and relates more particularly to a light source system employing a nonlinear crystal.
2. Description of the Related Art
Existing lasers do not provide lights with wavelengths covering the entire desired optical spectrum. Nonlinear crystals can be coupled to existing lasers to double the frequency of a laser, to sum or subtract the frequencies of two different lasers to produce a third frequency, or to parametrically generate a new frequency. As such, lights with wavelengths other than those of lights from existing lasers can be generated.
The frequency conversion processes include second harmonic generation, sum frequency generation (SFG), and difference-frequency generation (DFG). In the process of second harmonic generation (SHG), an incident beam at frequency ω is converted to radiation at the second harmonic frequency 2ω. In the process of sum-frequency generation (SFG), light with a frequency that is the sum of two other frequencies is generated. In the process of difference-frequency generation, light with a frequency that is the difference between two other frequencies is generated.
An RGB laser radiation source for generating red, green, and blue light beams using the above frequency conversion processes is disclosed in U.S. patent application Ser. No. 09/775,208. As shown in FIG. 1, the RGB laser radiation source includes a first laser radiation source 1 emitting a first beam with a wavelength of 1064 nm, which is partially frequency-doubled by SHG1 to generate green light with a wavelength of 532 nm and is partially directed to SFM1. A second laser radiation source 2 produces a second beam with a wavelength of 1550 nm, which is partially supplied to the SFM1 and mixed with the first beam to produce red light with a wavelength of 632 nm. SHG2 receives a portion of the second beam to generate a beam with a wavelength of 780 nm, which mixes with a portion of the first beam. The two beams are then sum-frequency mixed by SFM2 to generate blue light with a wavelength of 450 nm. The RGB laser radiation source uses several nonlinear crystals and two lasers to successfully generate red, green, and blue light beams. However, the design of such an RGB laser radiation source is complex and non-compact, and the adoption of too many nonlinear crystals may also increase the cost.
FIG. 2 shows another RGB source 10 disclosed in U.S. Pat. No. 7,489,437. The RGB source 10 has a wavelength conversion system 20 including a single NLO (nonlinear optical) unit 22 consisting of an optical parametric oscillator (OPO), which has a single periodically poled crystal 26 and is surrounded by input end-mirror 28I and output end-mirror 28O. A fiber-only laser 16 is used to provide light beam BN, which is converted by a non-linear optical process to generate red, green, and blue beams BR, BG, and BB. The OPO is usually configured to be singly resonant at an MIR idler wavelength. The input end-mirror 28I is antireflection (AR) coated at the wavelength of the light beam BN and high-reflection (HR) coated at red, green, blue, and MIR idler wavelengths. Likewise, the output end-mirror 28O is HR coated at the MIR idler wavelength and the wavelength of the light beam BN and is AR coated at red, green and blue wavelengths. The RGB source 10 needs to be built with high precision so that high conversion efficiency can be achieved. The RGB source 10 has a complex structure and is not easily built. In addition, the RGB source 10 uses expensive coated mirrors. Thus, it is expensive and not a suitable RGB source for consumer products.